OK, so you and your wife will buy the house with a mortgage of €40,000
He will "lend" you the deposit of €16,500.
You would make repayments of €300 a month over 15 years.
He would give you the €300 per month I presume.
He can lend or gift you the €16,500 - it won't matter as it's so far below the CAT limit.
You would be subject to income tax on the rental income of €300 per month.
You would be able to write off 80% of the interest element €166 per month, so you would pay tax on €140 a month.
You will end up owning the house. When you eventually sell it, you would be subject to CGT on any increase in value.
I wonder would it be ok for him to gift you €300 a month to pay the repayments? That way it would not be classified as rent and taxable? I suspect that Revenue might be ok with this in that they would look at the substance of the transaction.
Alternatively
Would the lender allow you and your father to buy the house jointly? That would be much easier for Revenue to understand.
It's a terrible pity that they would not give him a mortgage with you as guarantor. But so many guarantors in the past tried to escape their guarantees that lenders won't do them anymore.
Have you asked Pepper? They seem more flexible.
Brendan